How can it be ? The United States and Iran , sworn enemies for 35 years , are talking about working together to quell the al Qaeda-inspired insurgency sweeping northern Iraq .

Such cooperation sounds unthinkable . They are fierce adversaries on issues such as terrorism , Iran 's nuclear ambitions and Syria 's civil war .

Iranian leaders call the United States the `` Great Satan , '' while former President George W. Bush labeled Iran as part of an `` Axis of Evil . ''

It was headline news last year when their leaders spoke briefly by phone , the first contact at that level since the Islamic Revolution in 1979 drove the U.S.-backed Shah Mohammad Reza Pahlavi from power .

But should we be too surprised by this latest version of strange bedfellows , now known as `` frenemies '' in the modern vernacular ? There 's even an old saying of uncertain origin to define it -- the enemy of my enemy is my friend .

What 's going on ?

This time , the common enemy is the threat of a regional war based on sectarian battle lines , pitting Sunni and Shia Muslim governments and peoples against each other across the Middle East .

Only the Islamic State in Iraq and Syria -LRB- ISIS -RRB- and its al Qaeda backers would want that , it seems . They seek to establish a Sunni-dominated Islamic state stretching from Iraq to northern Syria .

`` We are now closer than ever to a regional war in the Middle East , '' said Paulo Sérgio Pinheiro , who heads the U.N. Independent International Commission of Inquiry on Syria . `` Events in neighboring Iraq will have grave and violent repercussions for Syria . The most dangerous aspect of these developments has been the rise of the sectarian threat , a direct consequence of the dominance of extremist groups like ISIS . ''

Shiite majority Iran seeks to protect Shia interests and power in Iraq , while the United States wants to see a stable Iraq after pulling its troops out of the country in 2011 to end its eight-year campaign that began by toppling Saddam Hussein from power .

Also , Iran has sounded a more positive tone toward the West since last year 's election of President Hassan Rouhani to succeed the more volatile Mahmoud Ahmadinejad .

On Wednesday , Rouhani pledged that Iran would do whatever was necessary to protect Shia holy sites in Iraq .

Where do things stand ?

U.S. Secretary of State John Kerry suggested possible collaboration with Iran , telling Yahoo News on Monday that `` we are open to discussions if there 's something constructive that can be contributed by Iran -- if Iran is prepared to do something that is going to respect the integrity and sovereignty of Iraq . ''

Then , the United States and Iran held `` very brief discussions '' about Iraq and the threat posed by ISIS on the sidelines of nuclear negotiations in Vienna , State Department spokeswoman Marie Harf said .

A senior State Department official said while Washington was open to engaging the Iranians , `` these engagements will not include military coordination or strategic determinations about Iraq 's future over the heads of the Iraqi people . ''

The official said on condition of not being identified that the discussion concerns the ISIS threat to `` many countries in the region , including Iran , '' as well as the need to support a more inclusive approach by the Iraqi government than the sectarian efforts by Prime Minister Nuri al-Maliki .

What 's in it for the United States ?

Defense Minister Chuck Hagel noted Wednesday that the United States and Iran cooperated early in the Afghanistan war . He told a congressional panel that `` we had worked with the Iranians on that western border of Afghanistan . ''

`` So there 's some history here of sharing common interests , '' Hagel said , citing `` significant differences '' with Iran , but adding that `` I do n't think these issues come neatly wrapped in geopolitical graduate school papers . ''

Even a conservative member of Congress who once advocated military strikes on Iran said Washington may need Tehran 's help .

`` I 'll talk to anybody to help our people from being captured or killed , '' Republican Sen. Lindsey Graham of South Carolina said . `` And this is a time where the Iranians in a small way might help . ''

Graham made clear he disapproved of Iran , calling authorities `` thugs and killers , '' then summed up the situation by saying : `` But we are where we are . ''

A host of experts agree , including Meghan O'Sullivan , a former deputy national security adviser during the Iraq war .

`` There is a political solution here that I think could be both in Iran 's interest and the U.S. interest , '' O'Sullivan said .

Whether the United States likes it or not , working with Iran on the Iraq crisis might be a necessary evil , retired Maj. Gen. James `` Spider '' Marks said .

`` There are necessary steps that we have to take with Tehran that we 've probably never taken before , and would prefer not to take , '' Marks said .

What 's the downside ?

Teaming up with Iran could certainly have its pitfalls .

The United States is wary of furthering Iran 's already considerable influence in Iraq . The Shiite Iranian regime is Maliki 's closest ally in the region , and a U.S.-Iranian partnership could alienate Iraq 's Sunni population as well as Sunni nations in the region such as Saudi Arabia that are U.S. allies .

Meanwhile , the United States does n't want to jeopardize international talks on Iran 's nuclear program that resumed this week .

The talks are intended to prevent Iran from developing a nuclear weapon , and the United States and Israel have repeatedly said they would use military means against Iran if necessary to achieve that outcome .

House Speaker John Boehner said Wednesday that the United States should `` absolutely not '' talk to Iranians about the Iraq crisis .

`` I can just imagine what our friends in the region , our allies will be thinking by reaching out to Iran at a time when they continue to pay for terrorists and foster terrorism not only in Syria , in Lebanon but in Israel as well , '' the Ohio Republican told reporters .

Will Iran and the U.S. work together on the ground ?

A senior security official in Baghdad said Iran has already sent about 500 Revolutionary Guard troops to help fight the ISIS militants .

Rouhani then denied that happened , but said he would be open to helping if asked , according to Iranian state TV .

A Pentagon spokesman said Monday that military coordination with Iran was not in the cards , similar to what the senior State Department official told CNN .

At the White House , spokesman Jay Carney said Obama has ruled out sending any U.S. combat troops to Iraq .

However , Graham noted the need to coordinate in other ways , such as coordinating possible U.S. air strikes on ISIS fighters if Obama orders them .

`` If we start flying airplanes , it makes some sense to talk to the Iranians about what we 're doing so they do n't shoot us down and we do n't bomb them , '' he said .

Back to Iraq : What can -- and should -- the U.S. do now ?

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Both Iran , United States want to quell the ISIS militant surge in Iraq

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Secretary of State Kerry suggests possible collaboration with Iran

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The issue raises questions in Congress

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Working with Iran might be a necessary evil , retired U.S. general says